Lamination of semi-rigid material between glass

ABSTRACT

A method is disclosed for producing superior optically clear laminates having a sheet of semi-rigid material encapsulated in a thermoplastic interlayer between layers of rigid material such as glass. Pre-cooled laminate elements are assembled at reduced temperature, allowing positioning of the sheet of semi-rigid material in the interlayer of the assembly with minimal wrinkling, formation of air bubbles or entrapment of moisture. Thereafter, the laminate assembly is maintained under vacuum for an extended period of time, substantially eliminating wrinkling, air bubbles and moisture from between the laminate elements as well as pre-forming the semi-rigid sheet. Bonding heat and pressure is then applied, first heating the assembly to &#34;tack&#34; the laminate elements together, and next applying heat and pressure for bonding the laminate elements. Superior optically clear laminates which result from the method are disclosed wherein a semi-rigid sheet of material is encapsulated in thermoplastic laminating material within an interlayer between sheets of rigid material such as glass.

BACKGROUND OF THE INVENTION

The present invention relates to a method for producing laminated glasssuch as employed for automobile windshields and other commercial andindustrial safety glass applications. In particular, the presentinvention relates to a method for producing a laminate comprising asheet of semi-rigid material encapsulated in a thermoplastic interlayerbetween layers of rigid transparent materials, such as glass or glasssubstitutes. The present invention further relates to the superiorlaminate produced thereby.

Various methods are known in the art for producing laminates comprisingflexible sheets of rigid transparent material, such as glass,polycarbonate, acrylic plastic, polyester and the like, havinginterlayers comprising sheets of material, such as polyvinyl butyral,polyurethane and the like. Typical of such methods are those disclosedby Foster, U.S. Pat. No. 3,406,086 at Cols. 5-7; Rieser et al, U.S. Pat.No. 3,808,077, at Cols. 17-18; Shumaker, U.S. Pat. No. 3,933,552; andthose discussed by Masuda, U.S. Pat. No. 4,358,329, at Col. 1; andMcMaster, U.S. Pat. No. 4,470,858, at Col. 1. Such bonding processesgenerally involve the application of heat and pressure, such as areavailable in oil autoclaves. Laminates have found wide application assafety glass due to their strength and preferable shatteringcharacteristics compared to plain or tempered glass.

Among the problems related to the bonding of laminates has been thepresence of moisture and air bubbles between laminate layers, and thewrinkling of flexible laminate layers, all of which optically distortthe final product. In addition, prior art bonding methods employing oilautoclaves may introduce oil between laminate layers along the edges ofthe laminate. To eliminate air, moisture and oil, methods for laminatingglass and thermoplastic sheet materials have been developed whichfurther employ peripheral evacuation of the laminate assemblies. Suchmethods are shown by Rieser et al, Shumaker, Curtz et al, U.S. Pat. No.4,543,283, Dlubak, U.S. Pat. No. 4,642,255 and Rase, U.S. Pat. No.4,647,327 which variously use vacuum bags or peripheral vacuum chambersto achieve peripheral evacuation. Evacuation of such vacuum bags orperipheral vacuum chambers, serves to draw air and moisture from betweenthe laminate layers, while the bags or chambers effectively create abarrier to oil penetration at the edges of those assemblies.

Nonetheless, the drawback of these bonding methods is that theapplication of heat and pressure required to bond the laminate layers isboth cumbersome and expensive. To avoid this drawback while maintainingstrength and other desirable characteristics, other bonding methodsoccurring at ambient temperatures have been developed in the prior artwhich involve the use of adhesive layers, such as disclosed by Masuda,LeGrand et al, U.S. Pat. No. 4,683,172, and McMaster. LeGrand et alfurther teach the use of a thin film of water containing surfactants asa means of preventing the formation of air bubbles between laminatelayers. Nonetheless, both strength and optical clarity of laminates,particularly the entrapment of water and air between laminate layers,continue to be of concern in the production of laminates.

Also of general relevance to the present invention are methods forencapsulating liquids or solids within laminate structures. Inparticular, methods for encapsulating liquid crystal materials betweenlaminates have been developed and are disclosed by IBM TechnicalDisclosure Bulletin, Vol. 15, No. 2, dated July 1972 and Ferrato, U.S.Pat. No. 4,418,102. In both of these references, an interlayer iscomprised of a seal which completely surrounds the liquid crystalmaterial, forming a sealed volume between laminate layers. In the IBMDisclosure, dielectric RF heating or ultrasonic energy is used to sealplastic layers such as polyvinyl chloride to create a volume which maythereafter be filled with liquid crystal material. The plastic layersforming the volume may further be sandwiched between glass plates. Inthe Ferrato patent, the seal between glass plates is comprised of athermoset product of resins, requiring the application of heat, thatforms a cavity which may be filled with liquid crystal material. Krugeret al, U.S. Pat. No. 4,469,408, and Ishii et al, U.S. Pat. No. 4,553,821also show full peripheral frames or seals for encapsulating liquidcrystal material in a laminate structure. Ishii et al employ heat and aheat sealing agent to seal the laminates and enclose a volume whereinliquid crystal material lies. Thus, in such applications, the liquidcrystal material fills a volume between laminates and does not, itself,adhere to other laminate layers to provide structural support.

The problems of optical distortion in a laminate structure due totrapped air, moisture and wrinkling of interlayers becomes of evengreater concern when dissimilar solid materials are joined to form aninterlayer in a laminate structure. In the Dlubak patent, a method isdisclosed for producing a laminated article having an interlayercomprising of fibrous layer encapsulated in a polyvinyl acetal, such aspolyvinyl butyral. The interlayer is sandwiched between two glassouterlayers. The fibrous layer is placed well within the peripheral edgeof the laminated article, presumably to avoid edge sealing problems, andis, for example, comprised of a woven lace cotton cloth. When placedbetween two glass outer layers and subjected to heat and pressure, thetwo sheets of polyvinyl butyral seal together to encapsulate the fibrouslayer. At the same time, the assembly is placed in a vacuum bag having apartial vacuum to remove trapped air and moisture between the laminae.The vacuum, high temperature and pressure are maintained for a period oftime during which the polyvinyl butyral is caused to flow through theinterstices of the woven lace cloth, and no detectable air bubbles arereported to remain. However, because the Dlubak patent relates to adecorative article which is translucent, rather than transparent,optical imperfections, for example due to wrinkling of the fibrouslayer, are not as critical as in applications where transparency isrequired, as with automobile windshields.

Thus, while the above references deal with encapsulating fluids anddecorative articles, neither of which are designed to enhance thestructural strength of the laminate, none address the encapsulation ofsemi-rigid sheet materials between laminates to form optically clearlaminate structures. Further, while rigid interlayers are shown byShumaker, the interlayer comprises an entire layer and is sandwichedbetween other layers, rather than being completely encapsulated.

Semi-rigid sheet materials are broadly defined as those which can bebent and flexed, but in so doing will tend to crease and fold. Forpurposes of illustration, such materials include, without limitationcardboard sheets; photography; or more pertinent to the presentinvention, certain plastic sheet materials such as polyester or Aclar™available from Allied Chemical Company. As well, semi-rigid materialsinclude composite sheet materials having semi-rigid properties, forexample liquid crystal coatings sandwiched between sheets of polyester,or electroluminescent coatings on electrodes sandwiched between sheetsof Aclar™.

Further developments in windshield technology have involved the use ofperipheral areas inward from the perimeter of the laminate for variousmaterials, such as optically opaque coatings, and electricallyconductive materials, for example as shown by Hurst, U.S. Pat. No.4,744,844.

Thus, a need exists for an effective method of producing optically clearlaminates, for example for use as windshields, glass roofs, moon roofs,side glass - any laminated glass part - wherein semi-rigid sheetmaterials are encapsulated within a laminate layer, but are locatedinward from the periphery, permitting other laminating materials to beused along peripheral edge areas. A need further exists for a method forencapsulating such materials and thereby provide additional strength tolaminate.

SUMMARY OF THE INVENTION

This need is satisfied by the present invention in which a sheet ofsemi-rigid material is encapsulated as an interlayer of a transparentlaminate. The method of the present invention eliminates air, moisture,wrinkles and creases from the laminate layers, and results in a strong,optically clear laminate structure. The method calls for the assembly ofpre-cooled laminate elements at reduced temperatures to mask andeliminate wrinkling, creasing and formation of air bubbles or moisturebetween the laminae. Pre-cooled below ambient temperatures to a desiredequilibrium temperature, the laminate elements are then assembled atsuch reduced temperatures. Of particular interest is the positioning ofa sheet of semi-rigid material as an interlayer of the laminateassembly. In accordance with this method, the semi-rigid material isultimately encapsulated in thermoplastic laminating material. Onceassembled, the cooled laminate assembly is degassed by evacuationmethods known in the art, such as vacuum bags or peripheral vacuumchambers connected to vacuum pumps. The cooled laminate assembly ismaintained under vacuum for a period of time depending on its shape,size and configuration. The laminate assembly, maintained under vacuum,is then bonded into a laminate by the application of heat and pressure.It is preferred to first heat the laminate assembly for a period of timewhich still under vacuum, for example by placing it in a heated chamber,which permits the thermoplastic laminating material to melt and becomesufficiently tacky to at least partially hold adjacent laminated layerstogether. Heat and pressure are next applied for a period of time, forexample by placing the laminate assembly in an autoclave, preferablyremoving the laminate assembly from under vacuum, although it may stillbe maintained under vacuum. Other methods known in the art may be usedto apply heat and pressure for this first step.

The pre-cooling of the laminate elements and assembly of those elementsat lowered temperatures stiffens the laminate elements, preventingwrinkling and creasing during handling. Moreover, cooling of thelaminate elements is believed to reduce the surface attraction andfriction between elements, allowing them to slip and conform to theirfinal shape without wrinkling, creasing or forming air bubbles as mayoccur in prior art methods.

The reduced friction between the elements is particularly beneficialwhen the elements are made to conform to curved laminate assemblies. Thepre-cooling temperature is not so cold that the laminate elements becomebrittle, stiff or incapable of conforming to desired shapes. Rather, thepre-cooling temperature renders semi-rigid materials more amenable tolamination. Evacuation of the assembly further prevents the formation ofair bubbles and removes moisture. It has been found that the longer theassembly is maintained in a vacuum, the better the result. This is ofparticular significance in encapsulating semi-rigid materials havingliquid crystal coatings to obtain optically clear curved laminateassemblies.

The present invention further provides a superior safety laminate whichis optically clear, free of moisture, air, wrinkles and creasing, andstrengthened by an encapsulated sheet of semi-rigid material. Thelaminate assembly is comprised of a first rigid transparant layer, afirst layer of thermoplastic laminating material, an interlayercomprising a sheet of semi-rigid material properly positioned andpreferably surrounded on its periphery with spacers of thermoplasticlaminating material, a second layer of thermoplastic material and asecond rigid transparent layer. By virtue of assembling and degassingthe laminate elements at lowered temperatures, trapped air, moisture,wrinkles and creasing are minimized, and the bonding process results ina laminate which exhibits both strength and high optical clarity. It maybe appreciated by one skilled in the art that high quality translucentor opaque laminates, or those using optically clear qualities of onlyone of the rigid transparent layers may be produced by the method of thepresent invention.

It is therefore an object of the present invention to provide a methodof assembling thermoplastic, semi-rigid, and rigid laminate at reducedtemperatures to mask and eliminate wrinkling, creasing, or formation ofair bubbles, whereby the semi-rigid material is encapsulated in thethermoplastic material. Another object of the present invention is toprovide a laminate having both strength and superior optical clarity byvirtue or encapsulating a transparent sheet of semi-rigid material inthe interlayer without the formation of air bubbles, moisture, wrinklingor creasing.

It is a further object of the invention to include in the interlayerareas contiguous with the outer periphery of the semi-rigid materialwhich areas may be used for other purposes, such as coatings, electricalconnections, and the like which are compatible with the thermoplasticlaminating material, while the overall laminate structure enjoys theadded strength provided by the presence of the semi-rigid materialtherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of the preferred embodiment ofthe laminate of the present invention.

FIG. 2 is a simplified cross-section of the laminate of the presentinvention.

FIG. 3 is a schematic perspective view of the assembled laminate in adegassing arrangement.

FIG. 4 is an alternative embodiment of the laminate of the presentinvention.

FIG. 5 is a second alternative embodiment of the laminate of the presentinvention.

FIG. 6 is a third alternative embodiment of the present invention.

FIG. 7 is a schematic perspective view of the assembled laminate in analternative degassing arrangement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the present invention, the elements of safety laminate 10of the present invention, shown in FIG. 1 are cooled by conventionalmeans from ambient temperature (generally 72° F.) to thermal equilibriumat a lower, cooler temperature. Whether in a cooled room, vault, box orthe like, the laminate elements are then assembled while at the lowertemperature. Further seen in FIG. 2, the laminate 10 is comprised of afirst layer 12 of rigid transparent material, preferably glass orpolycarbonate; a first layer 14 of thermoplastic laminating material,preferably a polyvinyl acetal; an interlayer 16, comprised of at leastone sheet of semi-rigid material 20, preferably polyester, andpreferably including one or more spacers 18 of the same thermoplasticlaminating material as first layer 14, which spacers 18 surround atleast part Of semi-rigid sheet 20; a second layer 24 of thermoplasticlaminating material of the same composition as first layer 14; and asecond layer 22 of rigid transparent material, preferably glass orpolycarbonate.

It has been found advantageous to assemble the elements at lower, coolertemperatures. In this assembly process, the semi-rigid sheet 20 ispositioned as desired within the laminate 10. In the preferredembodiment of FIG. 1, spacers 18 are added, preferably surrounding theentire periphery of semi-rigid sheet 20, as shown, encapsulating sheet20 and preventing the possibility of trapping air along an edge ofsemi-rigid sheet 20. In this regard, spacers 18 may be thicker thansemi-rigid sheet 20 to provide additional laminating material which canflow into gaps and voids during bonding. Spacer 18 may also be aone-piece skirt, fitted to surround semi-rigid sheet 20.

It has been found that pre-cooling to equilibrium and assembly oflaminate elements at cooler temperatures stiffens the laminate elements,particularly layers 14 and 24, semi-rigid sheet 20 and spacers 18. It isfurther believed that pre-cooling reduces the surface attraction andfriction between all the laminate elements. As a result, the laminateelements are allowed to slip, be positioned and conform to a final shapewithout wrinkling or creasing as occurs with laminate layers in theprior art methods. Further, the formation of air bubbles is alsoprevented. The cooler temperatures are below ambient (defined generallyas 72° F.), and preferably below 50° F. However, the temperature is notso cold that the laminate elements become brittle. In sum, the coolertemperature has the effect of rendering laminate elements, such assemi-rigid sheet 20, more amenable to lamination and encapsulation.

Once assembled, the laminate assembly is then clamped or held by knownmethods and enclosed, at least along its edges, in a sealed space orperipheral vacuum chamber as are known in the art. Such enclosure is inpreparation for degassing. Vacuum bags as described by Rieser et al,U.S. Pat. No. 3,808,077 or peripheral vacuum chambers such as thoseshown by Shumaker, U.S. Pat. No. 3,933,552 provide suitable sealedspaces communicating with the spaces between surfaces of the laminateelements.

As seen in FIG. 3, the assembly of laminate elements is degassed byevacuating sealed space 32 by conventional means such as mechanicalpumps 30. Evacuation of the assembly prevents the formation or retentionof air bubbles and draws moisture from between the laminate elements.Evacuation of sealed space 32 further encourages the laminate elementsto lie flat in a layered relationship. It has been found that the longerthe assembly is maintained under vacuum, the better are the resultsobtained. It is noted, however, that while the steps of pre-cooling andassembling at lowered temperatures, and degassing both improve theoptical clarity of resulting laminae, the step of assembling at lowertemperatures falls in the category of preventative measures, whereas thestep of degassing falls more in the category of corrective measures.

Once assembly at lower temperatures is completed and the step ofdegassing has begun, degassing is maintained for a length of time whichdepends on the shape, size and configuration of the laminate assembly.To determine the degassing time, a test may be run with periodic visualinspection of the laminate assembly during degassing to verify theabsence of air pockets or gaps therein. After such initial test runs,the degassing time can be determined for each particular type oflaminate assembly tested.

After degassing, the laminate assembly may be heated, preferably whilestill under vacuum, by known means, such as placing the laminateassembly in a heated chamber. The length of time for heating depends onthe size of the laminate assembly and characteristics of the heatedchamber. To determine this heating time, a test may be run with periodicvisual inspection of the laminate assembly to determine when thelaminating material has melted sufficiently to hold or tack the laminateassembly together. The laminate assembly need not be perfectly clear northe laminating material completely melted at this point. Typically, alaminate assembly may be maintained for 30 to 45 minutes in a chamberheated from 250° F. to 360° F., and emerge at a temperature in thegeneral range of 150° F. Again, however, the heating time varies withthe particular laminate assembly and may be determined in each case bytrial runs.

Thereafter, in accordance with the preferred method the laminateassembly may be removed from vacuum for application of additional heatand laminating pressure to bond the laminate elements. Desirabletemperature and pressures may be obtained by means known in the art suchas autoclaves. Although not preferred, pressure may alternatively beapplied by other mechanical means known in the art. Preferably, thislatter step in the bonding process occurs at a pressure of generally 100psig to 200 psig and at a temperature of from approximately 270° F. to300° F. over a period from 45 minutes to two hours.

Sealed space 32 is, preferably maintained under vacuum during initialapplication of bonding temperatures in order to continue to draw offmoisture and air. A shut off valve 34 or continued pumping withmechanical pump 30 can be used to seal or maintain the vacuum in sealedspace 32. Alternatively, methods of increasing the pressure in sealedspace 32 to prevent flow of thermoplastic laminating material as taughtby Rase, U.S. Pat. No. 4,647,327 may be used.

The preferred thermoplastic laminating material for practicing thepresent invention is polyvinyl butyral. However, other thermoplasticlaminating materials such as polyvinyl acetal or polyvinyl chloride maypermit use of other heating means such as dielectric RF heating orultrasound energy in connection with the application of known pressuremeans to bond the laminate elements. In addition, it is possible to addcoatings, electrical connections and the like between layers 14 and 24as well as between layers 12 and 14, 24, and 22, and, when used, spacers18 and layers 14 or 24.

In an alternative embodiment of the invention shown in FIG. 4, laminate10 may be comprised of first and second layers 12 and 22 of rigidtransparent material, as before; and a single interlayer 16' may becomprised of an extruded multilayer sheet 26 of semi-rigid sheetmaterial 20 sandwiched between and chemically linked to layers 14 and 24of thermoplastic laminating materials. Single interlayer 16' may furtherinclude spacers 18 of the same thermoplastic laminating material aslayers 14 and 24, which completely surround the periphery of multilayersheet 26. The method for assembling this alternative embodiment, asbefore, provides for precooling the laminate elements, and positioningand assembling the laminate elements at lower temperatures. Thereafter,the laminate assembly is degassed and subjected to the bondingtemperatures and pressures as described above. The laminate thus formedencapsulates the semi-rigid sheet 20 in thermoplastic material andachieves a safety laminate of similar high optical quality as enjoyedwith the preferred embodiment.

In further alternative embodiments shown in FIGS. 5 and 6, a pluralityof layers of semi-rigid sheet material 20' may be alternated with andencapsulated within layers 14', 24' of thermoplastic laminatingmaterial, preferably with spacers 18' in the manners aforesaid, andpositioned in a laminate assembly between rigid layers 12 and 22.

As it may be seen, the preferred laminate of the present invention has asheet of semi-rigid material encapsulated in a thermoplastic laminatingmaterial which is sandwiched between first and second rigid transparentlayers. In the preferred embodiment of FIG. 1, the thermoplasticlaminating material is polyvinyl acetal, specifically polyvinyl butyral;the first and second rigid layers are comprised of glass orpolycarbonates, and interlayer 16 comprises a semi-rigid sheet 20 ofpolyester and spacers 18 of thermoplastic laminating material,specifically, polyvinyl butyral. However, depending on the materialschosen, the semi-rigid sheet 20 in the preferred embodiment may or maynot be bonded to the thermoplastic laminating material of layers 14 and24, and spacers 18. However, by virtue of encapsulation, semi-rigidsheet 20 adds strength to the laminate assembly 10. Thus, the use ofother semi-rigid materials, bondable or not, such as electroluminescentmaterial is contemplated.

While certain representative embodiments and details have been shown anddescribed for purposes of illustrating the invention, it will beapparent to those skilled in the art that various changes in the methodsand articles disclosed herein may be made, such as including a pluralityof encapsulated, semi-rigid sheets between the first and second layersof rigid transparent material or using one or more translucent or opaquelayers, may be made without departing from the scope of the inventionwhich is defined in the appended claims.

What is claimed is:
 1. A method for producing a safety laminate,comprising the steps of:first, reducing the equilibrium temperature ofthe laminate elements from ambient temperature, said laminate elementsincluding:(a) a first rigid transparent layer; (b) a first layer ofthermoplastic laminating material; (c) an interlayer, comprising asemi-rigid material said semi-rigid material capable of bending andflexing, and tending to crease and fold thereby; (d) a second layer ofsaid thermoplastic laminating material; (e) a second rigid transparentlayer; and then, assembling said laminate elements into a laminateassembly at said reduced equilibrium temperature, with said interlayerpositioned between said first and second layers of said thermoplasticlaminating material; enclosing at least the edges of said laminateassembly in a sealed space; degassing said laminate assembly by reducingthe pressure of said sealed space by means of evacuating air therefrom;maintaining the reduced pressure of said sealed space for a period oftime; heating said laminate assembly and applying pressure thereto, tobond said laminate elements.
 2. A method as recited in claim 1 whereinsaid step of reducing comprises the step of reducing said equilibriumtemperature of the laminate elements to a temperature less than 70degrees Fahrenheit.
 3. A method as recited in claim 1 wherein said stepof reducing said equilibrium temperature comprises the step of reducingsaid equilibrium temperature of the laminate elements to a temperatureless than 50 degrees Fahrenheit.
 4. A method as recited in claim 1wherein said rigid transparent layers and said layer of thermoplasticlaminating material are of substantially the same size and wherein saidstep of assembling further includes the step of:positioning saidsemi-rigid material to peripherally surround said semi-rigid materialwith areas wherein said first and second layers of thermoplasticlaminating material may bond to encapsulate said semi-rigid material. 5.A method as recited in claim 1 wherein said rigid transparent layers andsaid layers of thermoplastic laminating material are of substantiallythe same size and said semi-rigid material is of a smaller size, andsaid interlayer further comprises one or more spacers of saidthermoplastic material; andwherein said step of assembling furthercomprises peripherally surrounding said semi-rigid material with saidspacers.
 6. A method as recited in claim 1 wherein said step ofenclosing comprises enclosing said entire laminate assembly in a vacuumbag.
 7. A method as recited in claim 1 wherein said step of enclosingcomprises enclosing said laminate assembly within a peripheralevacuation chamber disposed along the outer edges of said laminateassembly.
 8. A method as recited in claim 1 wherein said step ofdegassing comprises reducing the pressure of said sealed space with amechanical vacuum pump.
 9. A method as recited in claim 8 wherein saidstep of maintaining reduced pressure in said sealed space comprisesshutting a valve to isolate said sealed space from said mechanicalvacuum pump.
 10. A method as recited in claim 8 wherein said step ofmaintaining the reduced pressure in said sealed space comprisescontinued pumping of said sealed space with said mechanical vacuum pump.11. A method as recited in claim 1 wherein the reduced pressure in saidsealed space is maintained for a period of time, ending prior tocompletion of said step of heating and maintaining laminating pressure,and said method further comprising the step of:raising said pressure insaid sealed space to a pressure less than or equal to said laminatingpressure, whereby the outward flow of said thermoplastic laminatingmaterial from between said first and second rigid layers is inhibited.12. A method as recited in claim 1 wherein said step of heating saidlaminate assembly and maintaining laminating pressure comprises thesteps of:heating said laminate assembly for a heating period whilemaintaining reduced pressure of said sealed space; returning said sealedspace to ambient pressure; and applying laminating heat and pressure tosaid laminate assembly to bond said laminate elements.
 13. A method asrecited in claim 1 wherein said step of heating said laminate assemblyand maintaining laminating pressure comprises the step of:placinglaminate assembly in a chamber having an elevated temperature of fromgenerally 250° F. to generally 360° F. and ambient pressure forgenerally 30 minutes to generally 45 minutes, while maintaining saidreduced pressure of said sealed space; returning said sealed space toambient pressure; and placing said laminate assembly in an autoclave atan elevated temperature level of about 270° F. to 300° F. at an elevatedpressure of about 110 to 200 pounds per square inch gauge for a periodof from about 45 minutes to about two hours.
 14. A method as recited inclaim 1 wherein said step of assembling further includes the step ofslidably positioning said interlayer of semi-rigid material on thesurface of at least one of said first and second layers of saidthermoplastic laminating material to position said interlayer betweensaid first and second layers of said thermoplastic laminating material.15. A method as recited in claim 1 wherein said first and second rigidtransparent layers are curved, and said step of assembling comprisesassembling said laminate elements into a curved laminate assembly.
 16. Amethod for producing a safety laminate comprising the steps of:first,reducing the equilibrium temperature of a plurality of laminate elementsto a temperature below ambient temperature, said laminate elementscomprising:(a) a first rigid transparent layer; (b) a layer ofsemi-rigid material encapsulated within a layer of thermoplasticlaminating material, said layer of semi-rigid material having a lengthand width less than said first rigid transparent layer and saidsemi-rigid material capable of bending and flexing, and tending tocrease and fold thereby; and (c) a second rigid transparent layer havinga length and width substantially equal to that of said first rigidtransparent layer and then; assembling said laminate elements into alaminate assembly at said reduced equilibrium temperature, with saidrigid transparent layers being positioned on opposite sides of saidlayer of semi-rigid material; enclosing at least the edges of saidlaminate assembly in a sealed space; degassing said laminate assembly byreducing the pressure of said sealed space by means of evacuating airtherefrom; maintaining the reduced pressure of said sealed space for aperiod of time; and heating and applying pressure to said laminateassembly to bond said laminate elements.